Process and apparatuses have been previously developed for treating phosphoric acid to remove excess magnesium and calcium using fixed bed or moving folded bed cation exchange system. In the later connection, reference is made to the system disclosed in U.S. patent application Ser. No. 7,980, filed Jan. 31, 1979, and now U.S. Pat. No. 4,280,904, for "High Capacity Folded Moving Bed Ion Exchange Apparatus and Method for Treating Phosphoric Acid", which application has a common assignee with the present application, the assignee being American Petro Mart, Inc. An improvement in the system of that application is described in the application filed on even date with the present application, entitled "Method and Apparatus for Folded Moving Bed Ion Exchange System". In such systems, it is necessary to regenerate the cation exchange resin by removing the adsorbed magnesium and calcium. However, it has been found that the simultaneous elution of the magnesium and calcium can cause operational difficulties because of the low solubility of calcium sulfate in the aqueous sulfuric acid eluting solution. The calcium sulfate in the eluate tends to be saturated or supersaturated, and calcium sulfate precipitate forms within the resin beds being regenerated and in the pores of the resin beds.
Where the eluting solution is dilute sulfuric acid (15-20% H.sub.2 SO.sub.4) and a relatively high flow rate is employed through the resin bed being regenerated, the problems caused by calcium sulfate precipitation can be somewhat alleviated. However, it has been found more desirable to employ relatively concentrated sulfuric acid as the eluate (viz. 20-70% H.sub.2 SO.sub.4) and with high concentrations a slower flow rate through the bed is desirable to avoid subjecting the resin beads to fracturing concentrations of sulfuric acid. Therefore, it would be desirable to selectively remove cations, such as calcium ions, from the cation exchange resin prior to the removal of magnesium therefrom. The method of the present invention is believed to provide a novel and highly advantageous way of meeting this need. The method can also be used for processing solutions containing other cations, besides magnesium in admixture with calcium ions.